1. Field of the Invention
This invention generally relates to filter bottoms for use in filters for liquids, and more particularly to the structure of a filter block for underdrains which when assembled form a filter bottom for supporting a bed of finely divided filtering media. The filter bottom provides liquid flow conduits below the bed of filtering media, which conduits make possible the collection of filtered liquid and the distribution of gas and fluid backwash. The present invention is especially directed to a filter bottom providing a virtually complete, uniform distribution of gas and fluid backwash media throughout the entire filter bed while requiring a minimum amount of energy to thoroughly and evenly backwash the filtering media. The invention features special and novel aspects which are primarily directed to the establishment and maintenance of an usually high degree of uniformity of gas and fluid backwash distribution at minimum energy expense while markedly broadening the range of allowable backwash flow rates.
2. Description of the Prior Art
Several assemblies for filter bottoms are known in the art, particularly, assemblies having individual units called filter blocks, which are assembled together and interconnected with the appropriate supplies and drains. The filter blocks, when assembled provide an upper surface for supporting a filter media. The filter bottom upper surface is provided with apertures to allow the flow of filtered liquid from the filter media to pass into the filter bottom where conduits carry the filtered liquid from the filter. The conduits also serve to provide backwashing fluids, either gas or liquid or both, to the filter media for cleaning.
The filter bottom is covered with filtering media such as a bed of relatively coarse aggregate (the particles being too large to pass through the apertures in the top of the block), and several additional layers of graded material of larger to smaller and back to larger size farther above the filter bottom.
Conventionally, the liquid to be filtered, typically water, enters from above, passing downwardly through the filtering media, through the various layers of coarser particles, then through the apertures in the tops of the blocks, through conduits in the blocks, and out through a take-off flume. Gravity flow moves the liquid to be treated through the filter.
Periodically, the flow of liquid to be filtered is shut off and a washing medium is forced through the filter in reverse direction.
The wash medium (typically water) flows from the flume into the conduits which distribute it laterally away from the flume and from the conduits up through the separate upper chambers, the beds of particulate filtering material and out at the top, thereby carrying off deposited particles dislodged from the filter media. The backwash procedure usually includes first air backwashing before water backwashing. The air backwashing step loosens and separates the particles of the filter bed and then the subsequent water backwashing step fluidizes the bed and carries the deposited particles upward and from the bed. In many instances, air and water are used simultaneously. In all steps, the air and water flow through the bed must be uniformly distributed over the area of the bed. If the backwashing flow is not uniformly distributed, then the filter areas of low backwash velocity provide little backwashing effect and in areas of high fluid velocity, the flow will cause filter media to be carried upward and lost to disposal. Moreover, when the filter media is present in layers of different particulate materials, or different particulate size, non-uniform backwashing can cause undesired mixing of the particulate layers.
One type of the prior art filter blocks is shown in U.S. Pat. No. 3,110,667 to Stuppy. Each filter block includes a pair of parallel upper and a pair of parallel lower conduits, shown in cross-section in FIG. 6. Water from the filter passes through apertures in the top of each block into the upper conduits, then through ports in the floors of the upper conduits into the lower conduits. The liquid then flows from block to block to a flume. The Stuppy patent also proposes a liquid backwash wherein liquid is supplied to the lower conduits, passes upwards through the ports to the upper conduits and out the apertures to the filter media.
U.S Pat. No. 4,065,391 to Farabaugh discloses another configuration of filter block which does not include upper and lower conduits, but instead has an arrangement of parallel primary and secondary conduits positioned horizontally adjacent each other and separated by inclined walls. The inclined walls contain relatively smaller gas metering orifices and relatively larger liquid metering orifices with the liquid metering orifices positioned below the gas metering orifices. Backwash gas or liquid is supplied through the primary conduits, passes through the metering orifices into the secondary conduits, and from the secondary conduits into the bed of filter media. The gas metering orifices control the rate at which a backwash gas passes from the primary to the secondary conduits. The liquid metering orifices, and to a lesser extent the gas metering orifices, control the flow rate of a liquid backwashing medium.
The prior art filter block devices are deficient as to backwash operations. For example, the Stuppy device has relatively large liquid ports between the upper and lower conduits. If, although not disclosed in the Stuppy patent, a gas backwash were used with the device, the gas would be supplied to the lower conduits but then would pass easily through the first few of the relatively large liquid ports encountered to the upper conduits resulting in significantly unequal distribution of gas through the filter bottom. The uneven distribution of gas during such a backwash would serve to disrupt the filter media where too much gas flow occurs and to provide inadequate cleaning of the filter media where insufficient gas distribution occurs.
The Farabaugh device depends on a gas/liquid interface to control gas distribution during gas backwash. When gas backwashing begins, since the entire block and the filtering media above it is under water at that time, each of the conduits is essentially filled with water. When backwash gas is supplied to the primary conduit, a gas/liquid interface is formed as shown in FIG. 6 of the Farabaugh patent, and gas is metered to the secondary conduits via the gas metering orifices in the upper portion of the wall separating the primary and secondary conduits. However, the Farabaugh system can tolerate only a limited range of backwashing flow. If that limit is exceeded, the gas/liquid interface level is forced down to a point at which gas escapes into the secondary conduits through the oversized liquid metering orifices which are also located in the wall separating the primary and secondary conduits. Because of the rapid escape of the gas through the oversized liquid metering orifices, unequal distribution of the backwash gas, with its consequent disadvantages, occurs.
An additional problem with the Farabaugh device is that standing waves, created by a variety of phenomena during backwash of the filter such as a pressure shock from a sticking gas valve or other causes, can further limit the range of backwash flows. When such a standing wave is formed during backwash, it can reduce the level of the liquid/gas interface upstream of the wave so that the large liquid metering orifices are exposed to the gas flow, again creating unequal distribution of backwash gas.
It has also been found in the operation of filter bottoms as taught by Farabaugh that waves which frequently occur on the surface of the liquid in the filter, by changing temporarily the liquid pressure head over portions of the filter bottom, can thereby change the level of the liquid/gas interface in the primary conduit. This fluctuation in the level of the interface cyclically exposes then covers the oversized liquid metering orifices in the primary conduit to the gas above the interface, and, consequently, results in maldistribution of gas backwash as the oversized orifice is exposed.
The dependency of the Farabaugh design on a liquid/gas interface also limits the backwash rates which can be used during simultaneous gas and liquid backflush. The typical upper limit for simultaneous backwash in the Farabaugh device is approximately 5 standard cubic feet per minute ("SCFM") gas per square foot of filter bottom upper surface and 5 gallons per square foot of filter bottom per minute ("GSFM"). It has been proposed that raising one or both of the backwash rates simultaneously would increase scouring and cleaning of the filter media. However, the Farabaugh device, with its limitation of the gas/liquid interface can only accommodate simultaneous backwash rates within a limited range.
Finally, another problem with the Farabaugh device is that, because of the presence of the large liquid metering orifices in the primary conduit, it is particularly susceptible to problems stemming from non-level installation of the filter blocks. At column 5, line 14, Farabaugh states that the liquid metering orifices are preferably placed about 31/2 inches below the gas metering orifices. Accordingly, even slight errors in installation of the filter blocks can markedly reduce the vertical distance between the lowest gas orifice and the highest liquid orifice which share the same gas/liquid interface. This non-level installation significantly reduces the safe operating range for gas backwashing to avoid escape of the gas through the liquid orifices. Although modifications of the Farabaugh device have been proposed, extending the vertical distance between the gas and liquid metering orifices to almost 9 inches, the same problems still occur.
The previously known filter block arrangements suffer from sensitivity to non-level alignment of the blocks. Even small divergences from level alignment of the blocks leads to significantly non-uniform backwashing performance, particularly when the backwash medium is a gas. Accordingly, it is an object of this invention to provide a filter underdrain block structure which when assembled and arranged to form a filter bottom, maximizes uniform distribution of backwashing gas and backwashing liquid, fluidizes the filtration media over the underdrain block, dislodges dirt and debris entrapped in the filter media, and thoroughly cleans the media.
It is another object of the present invention to provide a filter block underdrain with a reduced sensitivity to non-level block alignment, particularly with regard to gas backwashing. A further object of the invention is to provide a filter block weighing less than conventional filter blocks yet having a good structural integrity thereby being easier to handle and easier to install than conventional clay filter blocks.
Other objects and advantages of the present invention will be apparent from the following detailed description and from the appended drawings.